Field of the Invention
The invention relates to a fitting or a valve, in particular for shutting off a flow, including a threaded part being axially displaceable and fixed against relative rotation in a housing, a threaded part being axially resiliently and rotatably supported in the housing, a braking device having at least two friction surfaces facing toward one another, being pressed against one another with a braking force in order to brake the rotary motion of the rotatable threaded part by means of an axial displacement and form a braking surface, if a predetermined rated force upon the threaded part being fixed against relative rotation is exceeded, one of the friction surfaces facing toward one another being connected to the rotatable threaded part in such a manner as to be fixed against relative rotation, and a cup spring assembly having at least one cup spring for resiliently supporting the rotatable threaded part in at least one axial direction.
Fittings for shutting off a flow often have an axially displaceable spindle that is provided with a closure piece, such as a valve cone or valve plate, and which is moved by a spindle nut being rotatably supported in a housing. As the closure piece moves into the seat, a predetermined torque is necessary. That torque depends on the designated pressure in the flow to be shut off, on the spindle geometry, and on the coefficient of friction in the spindle nut thread. In the case of a high-pressure valve with a rated width of 25 mm, for instance, a rated torque value (minimum value) of approximately 30 Nm is typical. However, after a long period of service, corrosion can considerably increase the coefficients of friction in the spindle nut thread. Correspondingly higher torques, for instance approximately 80 Nm, are then necessary to loosen the spindle nut.
In that case, an actuator for the fitting must be constructed in such a way that it can bring such a high loosening moment to bear even under unfavorable operating conditions. In the case of an electric motor actuator, by way of example such a moment may be a low supply voltage resulting from fluctuations in the mains voltage. However, at high mains voltage, with a cold winding in the electric motor, a positive motor production tolerance, and high fitting rigidity, the actuator can develop a much higher torque if it is not turned off as a function of torque. If there is an unfavorable cooperation among all of the tolerances, this torque can rise to 300 Nm, for instance. If the torque-dependent shutoff fails, then in an unfavorable case a torque can occur that is ten times the rated torque. The spindle is then severely overloaded and may buckle.
Published European Application No. 0 193 776 B1, corresponding to U.S. Pat. No. 4,770,390, discloses a fitting in which the torque made available by the actuator for generating a spindle force is already reduced during the adjusting motion. To that end, a brake bush is provided that is supported by slide surfaces on its ends on a brake housing and which, after an idle rotation angle of approximately 330.degree., is rotated along with the spindle nut. When the spindle nut is rotated backward again, the braking is omitted over the idle angle range, so that the torque generated by the actuator is fully available for loosening the spindle clamped in the nut thread. The braking moment created in the brake already becomes operative before the terminal position is reached and is proportional to the spindle force and to the coefficient of friction of the slide surfaces. By way of example, the brake can be constructed in such a way that as it moves into the terminal position it dissipates approximately 75% of the drive torque. Using the above numerical example, this means that the drive would have to be constructed not for a minimum torque of 30 Nm, but rather for one of 120 Nm. This makes it too expensive and too heavy in many cases. Moreover, the torques that arrive if shutoff fails under unfavorable conditions, are likewise correspondingly increased.
FIG. 4 of German Published, Non-Prosecuted Application 33 14 781 A1, corresponding to U.S. Pat. No. 4,546,281, discloses a fitting in which the spindle nut is additionally braked only after a predetermined spindle force is exceeded. This is achieved by supporting the spindle nut axially resiliently in the fitting housing and providing it with a runup ring which, if a predetermined spindle force is exceeded, is pressed against a brake bush being supported in the housing and including annular springs. As a result of the braking moment generated between the runup ring and the brake bushes, a braking action is generated so that only some of the torque made available by the drive becomes operative on the spindle. Such provisions lessen the load effectively on the spindle in the event of a so-called shutoff failure.
However, in that known fitting only some of the total spindle force becomes operational as a braking force that generates a braking torque. Under unfavorable conditions, the braking moment generated in this way may be inadequate to prevent an overload of the spindle.
It is accordingly an object of the invention to provide a fitting for shutting off a flow, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which is a fitting with a braking device for controlling shutoff failure, in which the effective braking force in the braking device is as high as possible.